Less allocated objects on each request

How many? Using `memory_profiler` and a Rails app (codetriage.com), master uses:

```
rack/lib x 7318
```

After this patch, the app uses:

```
rack/lib x 4598
```

Or `(7318 - 4598) / 7318.0 * 100 # => 37.16` % fewer objects __PER REQUEST__.

To do this, I extracted really commonly used strings into top level Rack constants. It makes for a bit of a big diff, but I believe the changes are worth it. 

Running benchmark/ips against the same app, I'm seeing a performance host of `2~4%` across the entire app response. This doesn't just make Rack faster, it will make your app faster.

While we could certainly go overboard and pre-define ALL strings as constants, that would be pretty gnarly to work with. This patch goes after the largest of the low hanging fruit.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/rack/sendfile.rb b/lib/rack/sendfile.rb
index 8e2c3d67..4a9b428b 100644
--- a/lib/rack/sendfile.rb
+++ b/lib/rack/sendfile.rb
@@ -116,7 +116,7 @@ module Rack
         when 'X-Accel-Redirect'
           path = F.expand_path(body.to_path)
           if url = map_accel_path(env, path)
-            headers['Content-Length'] = '0'
+            headers[CONTENT_LENGTH] = '0'
             headers[type] = url
             obody = body
             body = Rack::BodyProxy.new([]) do
@@ -127,7 +127,7 @@ module Rack
           end
         when 'X-Sendfile', 'X-Lighttpd-Send-File'
           path = F.expand_path(body.to_path)
-          headers['Content-Length'] = '0'
+          headers[CONTENT_LENGTH] = '0'
           headers[type] = path
           obody = body
           body = Rack::BodyProxy.new([]) do